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Plant Health Management: Soil Fumigation
Plant Health Management: Soil Fumigation DO Chellemi, US Department of Agriculture, Fort Pierce, FL, USA Published by Elsevier Inc.
Introduction Soil fumigation is a form of soil disinfestation that involves the application of volatile chemical compounds (fumigants) before planting to rid the soil of plant pathogens and pests. Other forms of soil disinfestation include steam sterilization, solarization (the use of clear plastic to heat soil), flooding, and anaerobic soil disinfestation (a combination of flooding and organic amendments). Soil fumigation is practiced on a wide variety of agronomic, fruit, nursery, ornamental, turf, and vegetable crops to control a broad spectrum of soilborne pests including plant pathogens, nematodes, insects and weeds. To be effective, chemicals used for soil fumigation must be extremely toxic to a wide range of organisms encompassing actinomycetes, bacteria, fungi, oomycetes, insects, nematodes, and plants (both seeds and vegetative material). Many popular chemical fumigants are halogenated organic compounds that disrupt basic cellular structure or functions via methylation of protein sulfhydryl groups, interference of the cytochrome-mediated electron transport chain, and/or direct attack on amino and hydroxyl groups. Soil fumigants are unique chemical pesticides. They are highly volatile, enabling them to disperse rapidly through the soil profile following application as a liquid. To achieve this, chemical fumigants should include a number of characteristics, including low boiling points and high vapor pressures; and they should posses a strong affinity to move through air spaces within the soil profile. Henry's law (KH) constant is employed to measure the partition potential of a compound between the air and water phases. In general, KH values410−4 imply that compounds will diffuse more rapidly in the vapor phase and thus will move quickly through the soil profile when the soil is below 50% saturation. Soil fumigants can be applied either through the irrigation system (drip or overhead) or directly into soil via metal shanks attached to a farm implement (Figure 1). Details on the principals and theory of soil fumigation have been reviewed extensively by Goring (1962) and Lembright (1990). Soil fumigation is a popular form of pest control because one chemical application can effectively replace the need for combined applications of soil fungicides, bactericides, nematicides and herbicides, resulting in a substantial financial savings to growers. Soil fumigation is also popular because one simple application can eliminate the need for more complicated knowledge-based pest management programs that require additional information acquisition and processing, decision making, and long-term land management programs such as crop rotation. Initially, an inexpensive method of soil disinfestation, soil fumigant costs have risen dramatically over the past 15 years and in the US can reach US$2766 per hectare and account for 15% of the total operating costs for vegetable production (University of Florida, 2009). Thus, soil fumigation is now typically restricted to high value, intensively
456
managed crop production systems where large monocultures are repeatedly planting in same location, precluding the use of crop rotations or fallow periods to mitigate the build-up of soilborne pests. Another interesting side note is the contribution of soil fumigation, particularly using the chemical methyl bromide, to agricultural and ecological research. Through comparison of yields to fumigated soil, it has been used to access and optimize the performance of more sustainable crop management practices (Cook et al., 2002). It has also been used in ecological field research to experimentally test ecological theories such as island biogeography (Simberloff and Wilson, 1969).
History The first documented example of soil fumigation using volatile organic compounds dates from 1869, where Baron Paul Thenard applied carbon disulfide (CS2) to French vineyards infested with the root parasite Phylloxera vastatrix (Rohart, 1876; Rommier, 1876). Eventually, over 50 000 ha of European vineyards where fumigated by the end of the nineteenth century. The origin of many chemical fumigants began as waste byproducts of industrial reactions. A mixture of dichloropropene and dichloropropane (DD) was produced as a byproduct of the production of allyl chloride by Shell Company and presented a disposal problem until its nematicial activity was discovered by Walter Carter in Hawaii (Carter, 1943; Caswell and Apt, 1989). The fumigant methyl bromide, initially manufactured as a flame retardant in the 1920s, was produced as a precursor to safer fire retardants such as BCF and Halon 1211. The popular soil fumigant chloropicrin (trichloronitromethane, CCl3NO2) had a more
Figure 1 Traditional shank application of soil fumigants practiced in the southeastern US.
Encyclopedia of Agriculture and Food Systems, Volume 4
doi:10.1016/B978-0-444-52512-3.00250-3
Plant Health Management: Soil Fumigation
insidious origin before its introduction to agriculture. Chloropicrin, produced by the reaction of picric acid and bleach, remained largely a laboratory curiosity until World War I when it gained widespread use in chemical warfare, both a lachrymator and a lethal gas. At the closure of World War I, the US had stockpiled an estimated 36 million pounds of chloropicrin (Roark, 1934) and agricultural scientists begin to assess the surplus chloropicrin for insecticidal properties (Cotton and Young, 1929; Moore, 1918). By the late 1950s, soil fumigation with mixtures of chloropicrin and methyl bromide had become the standard soil disinfestation practice for high-value crops such as tomato and strawberry (Geraldson et al., 1965; Wilhelm et al., 1961), replacing other less effective soil fumigants. For 50 years, methyl bromide was widely used as a soil fumigant due to its low cost, ease of handling, performance under a wide range of soil and environmental conditions, low phytotoxicity, and broad range of activity. Globally, it was viewed as indispensable wherever intensive agriculture was practiced (Vanachter, 1975), and in the US a 1990s economic study estimated that a ban on the use of methyl bromide would cost producers and consumers more than US$1 billion annually (Anonymous, 1993).
Biological, Environmental, and Human Impacts The same combination of acute toxicity and high volatility that make fumigants an effective soil pesticide can also increase the risk of field worker and bystander exposure, contamination of ground and surface water, and environmentally damaging atmospheric emissions, creating a contentious source of conflict between the agricultural industry and consumer and environmental advocates. The registration of ethylene dibromide, at the time one of the most abundantly used nematicides in the world, was suspended by the US Environmental Protection Agency (USEPA) after it was identified in ground waters of Arizona, California, Connecticut, Florida, Georgia, Massachusetts, and Washington (Federal Resister, 1983). A contaminant of DD was detected in municipal drinking wells located near former pineapple production fields on the Hawaiian islands of Kauai, Oahu, and Maui, prompting a voluntary withdrawal by the manufacturer (Oki and Giambelluca, 1987). Use of the popular soil fumigant 1,2-dibromo-3chloroprane (DBCP) was suspended by the USEPA after it was discovered that one-third of male workers at a DBCP manufacturing plant in California were sterile (Thrupp, 1991). Later, a massive sterilization of approximately 1500 banana plantation workers in Costa Rica from DBCP was also documented (Thrupp, 1991). DBCP was also detected in the ground waters of Arizona, California, Hawaii, Maryland, and South Carolina. In 1991, metam sodium gained national attention when a train derailed spilling over 43 000 l into the Sacramento River, causing catastrophic damage to wildlife and the river ecosystem (Cantara Trustee Council, 2007; Brett et al., 1995). Methyl bromide was identified as a significant ozone-depleting compound, and agricultural soil fumigation was implicated as a major anthropogenic source (Yagi et al., 1993; WMO, 1994). Under obligations of the US Clean Air Act and the Montreal Protocol on Substances that Deplete the Ozone Layer (an international stratospheric ozone protection treaty) a
457
scheduled phase-out of production and sale was initiated in developed countries (USEPA, 1993; UNEP, 1995). Since 1999, production and importation of methyl bromide in the US has declined from 25 500 to 434 metric tons. In Europe, registrations for 1,3-dichloropropene (1,3-D) and chloropicrin were canceled and production and sale of both fumigants were phased out between 2008 and 2012.
Current and Future Status In the US, the number of chemical compounds registered for use in soil fumigation has diminished to chloropicrin, dimethyhl disulfide (DMDS), methyl isothiocyanate (MITC) generators (dazomet, metam potassium, and metam sodium), and 1,3-D. Their chemical properties have been extensively reviewed by Ajwa et al. (2003) and Ruzo (2006), and a generalized summary of their maximum-use rates and relative effectiveness to methyl bromide is available (Noling et al., 2010). A comprehensive reevaluation of chloropicrin, DMDS, and the MITC generating compounds by the USEPA in 2010 led to additional regulations place on their use to increase protection of agricultural workers and bystanders. Some of the new regulations include increased buffer zones of untreated soil between the application sites and neighboring residences, detailed site-specific fumigant management plans, emergency preparedness and response notification, and formal applicator training programs administered by the fumigant registrants (USEPA, 2013). The additional compliance assistance and assurance measures will dramatically restrict the ability to apply soil fumigants, particularly in urbanized areas. Recent trends in soil fumigation have focused on developing a set of good agricultural practices (GAPs) for fumigant applicators that, when adhered to, enhance the pest control efficacy of alternative chemical fumigants and minimize their environmental and human health impacts. Key features of GAPs include the use of improved application methods and technology to reduce fumigant application rates, improve the precision of fumigant placement within the soil profile, and restrict their atmospheric emissions. For example, low disturbance fumigant application equipment was developed using small vertical colters mounted forward of a soil bed press to improve the precision of fumigant delivery without disturbing the soil profile (Figure 2). The combined use of low-disturbance application equipment and a soil surface seal reduced cumulative atmospheric emission of 1,3-D by 21% and chloropicrin by 18% when compared to traditional shank applications (Chellemi et al., 2012). An under-bed fumigator using winged steel shanks mounted behind large vertical colters was developed to deliver soil fumigants underneath existing plastic mulched beds without disturbing the integrity of the beds (Figure 3). Significant control of plant parasitic nematodes and weeds with reduced fumigant application rates was achieved when the under-bed applications were made 7 days after planting beds were covered with plastic film (Chellemi and Mirusso, 2006). New formulations of plastic film have been developed and used as tarps to retain fumigants in the soil following application by using layers of ethylene vinyl alcohol resins, nylon, or other polyaminides. A standardized approach for estimating their permeability to soil fumigants was developed and used to assess
458
Plant Health Management: Soil Fumigation
their performance under a range of field and application conditions (Papiernik et al., 2011). Total cumulative atmospheric emissions of chloropicrin and MITC were reduced to less than 5% of the material applied when low-disturbance application technology was combined with improved plastic films (Chellemi et al., 2010). Also included in GAPs are the selective inclusion of specific soil and environmental conditions that mitigate the impact of fumigant emissions. They include soil moisture above field capacity and good soil tilth. Soil tilth is
Figure 2 Use of vertical colters to improve the precision of fumigant placement within the planting bed and to minimize disturbance of the soil profile.
defined as the physical condition of soil as related to its ease of tillage, fitness as a seedbed, and its promotion of seedling emergence and penetration (Karlen, 2005). An illustrated comparison of two fumigated planting beds with contrasting levels of soil tilth is presented in Figure 3. Ranked in order of impact on the retention of fumigants in the soil are soil water content, soil tilth, the type of plastic film, and soil texture (Chellemi et al., 2011). When GAPs are following during the fumigant application process, many soil fumigants can achieve a spectrum of pest control similar to methyl bromide while maintaining a high level of marketable yields (Ajwa et al., 2002; Chellemi and Mirusso, 2006; Gilreath et al., 1999; Locascio et al., 1997; MacRae et al., 2010).
Figure 3 Precision placement of fumigants under plastic mulched beds using vertical colters and winged steel shanks.
Figure 4 A comparison of raised plastic mulched beds prepared in soils with good tilth (left) and poor soil tilth (right).
Plant Health Management: Soil Fumigation
By definition, soil disinfestation procedures are not sustainable because reinfestation of treated soil occurs during crop production, necessitating the reapplication of soil disinfestation procedures for future crops. Future trends in soil fumigation research will include ways to mitigate the impact of chemical fumigants on nontarget beneficial soil microbial communities as a means of extending the time intervals between fumigant applications (Figure 4).
See also: Advances in Pesticide Risk Reduction. Plant Health Management: Crop Protection with Nematicides
References Ajwa, H.A., Klose, S., Nelson, S.D., et al., 2003. Alternatives to methyl bromide in strawberry production in the United States of America and the Mediterranean region. Phytopathologia Mediterranea 42, 220–224. Ajwa, H.A., Trout, T., Mueller, J., et al., 2002. Application of alternative fumigants through drip irrigation systems. Phytopathology 92, 1349–1355. Anonymous, 1993. The biological and economic assessment of methyl bromide. The National Agricultural Pesticide Impact Assessment Program (NAPIAP). Washington, DC: USDA. Cantara Trustee Council, 2007. Final report on the recovery of the upper Sacramento River − subsequent to the 1991 Cantara spill, CA, USA: Cantara Trustee Council. 53 pp. Brett, M.T., Goldman, C.R., Lubnow, F.S., et al., 1995. Impact of a major soil fumigant spill on the planktonic ecosystem of Shasta Lake, California. Canadian Journal of Fisheries and Aquatic Species 52, 1247–1256. Carter, W., 1943. A promising new soil amendment and disinfectant. Science 97 (2521), 383–384. Caswell, E.P., Apt, W.J., 1989. Pineapple nematode research in Hawaii: Past, present, and future. Journal of Nematology 21 (2), 147–157. Chellemi, D.O., Ajwa, A.A., Sullivan, D.A., et al., 2011. Soil fate of agricultural fumigants in raised-bed plasticulture systems in the southeastern United States. Journal of Environmental Quality 40, 1204–1214. Chellemi, D.O., Ajwa, H.A., Sullivan, D.A., 2010. Atmospheric flux of agricultural fumigants from raised-bed, plastic-mulch crop production systems. Atmospheric Environment 44, 5279–5286. Chellemi, D.O., Mirusso, J., 2006. Optimizing soil disinfestation procedures for fresh market tomato and pepper production. Plant Disease 90, 668–674. Chellemi, D.O., Mirusso, J., Ajwa, H., Sullivan, D.A., Unruh, J.B., 2012. Fumigant persistence and emission from soil under multiple field application scenarios. Crop Protection 43, 94–103. Cook, R.J., Weller, D.M., El-Banna, A.Y., Vakoch, D., Zhang, H., 2002. Yield responses of direct-seeded wheat to rhizobacterial and fungicide seed treatments. Plant Disease 86, 780–784. Cotton, R.T., Young, H.D., 1929. The use of carbon dioxide to increase the insecticidal efficacy of fumigants. Proceedings of the Entomological Society of Washington 31, 97–102. Geraldson, C.M., Overman, A.J., Jones, J.P., 1965. Combination of high analysis fertilizers, plastic mulch, and fumigation for tomato production on old agricultural land. Proceedings − Soil & Crop Science Society of Florida 25, 18–24. Gilreath, J.P., Noling, J.W., Locascio, S.J., Chellemi, D.O., 1999. Effect of methyl bromide, 1,3-dichloropropene þ chloropicrin with pebulate and soil solarization on soilborne pest control in tomato followed by double cropped cucumber. Proceedings of the Florida State Horticultural Society 112, 292–297. Goring, C.A.I., 1962. Theory and principals of soil fumigation. Advances in Pest Control Research 5, 47–84. Karlen, D.I., 2005. Tilth. In: Hillel, D. (Ed.), Encyclopedia of Soils in the Environment. San Diego, CA: Elsevier Academic Press, pp. 168–174. Lembright, H.W., 1990. Soil fumigation: Principals and application technology. Journal of Nematology 22 (4S), 632–644.
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Locascio, S.J., Gilreath, J.P., Dickson, D.W, et al., 1997. Fumigant alternatives to methyl bromide for polyethylene-mulched tomato. HortScience 32, 1208–1211. MacRae, A., Noling, J., Snodgrass, C., 2010. Maximizing the efficacy of soil fumigant applications for raised-bed plasticulture systems of Florida Gainesville, Florida: University of Florida, IFAS Publ. HS1179. Moore, W., 1918. Fumigation with chloropicrin. Journal of Economic Entomology 11, 357–362. Noling, J.W., Botts, D.A., MacRae, A.W., 2010. Alternatives to methyl bromide soil fumigation for Florida vegetable production. In: Olson, S.A., Simonne, E. (Eds.), Vegetable Production Handbook for Florida. FL, USA: Citrus and Vegetable Magazine, University of Florida, Food360, pp. 43–50. Oki, D.S., Giambelluca, T.W., 1987. DBCP, EDB, and TCP contamination of ground water in Hawaii. Ground Water 25, 693–702. Papiernik, S.K., Yates, S.R., Chellemi, D.O., 2011. A standardized approach for estimating the permeability of plastic films to soil fumigants under various field and environmental conditions. Journal of Environmental Quality 40, 1375–1382. Roark, R.C., 1934. Bibliography of chloropicrin, 1848−1932. Washington, DC: US Department of Agriculture Miscellaneous publication No. 176. 88 pp. Rohart, F., 1876. Ou en est la question Phylloxera. Journal d'Agriculture Pratique 40 (2), 721–726. Rommier, A., 1876. Les divers proceḋ eṡ essayeṡ jusqu'a present pour combattre le Phylloxera. Journal d'Agriculture Pratique 40 (2), 762−766, 856-857. Ruzo, L.O., 2006. Physical, chemical and environmental properties of selected chemical alternatives for the pre-plant use of methyl bromide as soil fumigant. Pest Management Science 62, 99–113. Simberloff, D.S., Wilson, E.O., 1969. Experimental zoogeography of islands: The colonization of empty islands. Ecology 50, 278–296. Thrupp, L.A., 1991. Sterilization of workers from pesticide exposure: The causes and consequences of DBCP-induced damage in Costa Rica and beyond. International Journal of Health Services 21, 731–757. University of Florida. 2009. International agricultural trade and policy center. FL, USA: University of Florida. Available at: http://www.fred.ifas.ufl.edu/iatpc/budgets. php (accessed 15.01.14). United Nations Environment Programme (UNEP), 1995. Montreal Protocol on substances that deplete the ozone layer. 1994 Report of the methyl bromide technical options committee. Nairobi, Kenya: United Nations Ozone Secretariat. United States Environment Protection Agency (USEPA), 1993. 40 CPR Part 82, protection of stratospheric ozone: Final rule making. Federal Register 58 (236), 65018–65062. USEPA, 2013. The phaseout of methyl bromide. USEPA. Available at: http://www. epa.gov/ozone/mbr/index.html (accessed 03.05.14). Vanachter, A., 1975. Fumigation against fungi. In: Mulder, D. (Ed.), Soil disinfestation. New York: Elsevier, pp. 163–184. Wilhelm, S., Storkan, R.C., Sagen, J.E., 1961. Verticillium wilt of strawberry controlled by fumigation of soil with chloropicrin and chloropicrin-methyl bromide mixtures. Phytopathology 51, 744–748. WMO, 1994. Scientific assessment of ozone depletion: 1994. Executive summary. World Meteorological Organization Global Ozone Research and Monitoring Project − Report No. 37. Geneva: WMO. Yagi, K., Williams, J., Wang, N.Y., Cicerone, R.J., 1993. Agricultural soil fumigation as a source of atmospheric methyl bromide. Proceedings of the National Academy of Sciences of the USA 90, 8420–8423.
Relevant Websites http://www.mbao.org/ Methyl Bromide Alternatives Outreach. http://ozone.unep.org/new_site/en/montreal_protocol.php United Nations Environment Programme. http://www.epa.gov/opp00001/reregistration/soil_fumigants/index.htm United States Environmental Protection Agency. http://www.epa.gov/ozone/mbr/ United States Environmental Protection Agency.
Introduction Soil fumigation is a form of soil disinfestation that involves the application of volatile chemical compounds (fumigants) before planting to rid the soil of plant pathogens and pests. Other forms of soil disinfestation include steam sterilization, solarization (the use of clear plastic to heat soil), flooding, and anaerobic soil disinfestation (a combination of flooding and organic amendments). Soil fumigation is practiced on a wide variety of agronomic, fruit, nursery, ornamental, turf, and vegetable crops to control a broad spectrum of soilborne pests including plant pathogens, nematodes, insects and weeds. To be effective, chemicals used for soil fumigation must be extremely toxic to a wide range of organisms encompassing actinomycetes, bacteria, fungi, oomycetes, insects, nematodes, and plants (both seeds and vegetative material). Many popular chemical fumigants are halogenated organic compounds that disrupt basic cellular structure or functions via methylation of protein sulfhydryl groups, interference of the cytochrome-mediated electron transport chain, and/or direct attack on amino and hydroxyl groups. Soil fumigants are unique chemical pesticides. They are highly volatile, enabling them to disperse rapidly through the soil profile following application as a liquid. To achieve this, chemical fumigants should include a number of characteristics, including low boiling points and high vapor pressures; and they should posses a strong affinity to move through air spaces within the soil profile. Henry's law (KH) constant is employed to measure the partition potential of a compound between the air and water phases. In general, KH values410−4 imply that compounds will diffuse more rapidly in the vapor phase and thus will move quickly through the soil profile when the soil is below 50% saturation. Soil fumigants can be applied either through the irrigation system (drip or overhead) or directly into soil via metal shanks attached to a farm implement (Figure 1). Details on the principals and theory of soil fumigation have been reviewed extensively by Goring (1962) and Lembright (1990). Soil fumigation is a popular form of pest control because one chemical application can effectively replace the need for combined applications of soil fungicides, bactericides, nematicides and herbicides, resulting in a substantial financial savings to growers. Soil fumigation is also popular because one simple application can eliminate the need for more complicated knowledge-based pest management programs that require additional information acquisition and processing, decision making, and long-term land management programs such as crop rotation. Initially, an inexpensive method of soil disinfestation, soil fumigant costs have risen dramatically over the past 15 years and in the US can reach US$2766 per hectare and account for 15% of the total operating costs for vegetable production (University of Florida, 2009). Thus, soil fumigation is now typically restricted to high value, intensively
456
managed crop production systems where large monocultures are repeatedly planting in same location, precluding the use of crop rotations or fallow periods to mitigate the build-up of soilborne pests. Another interesting side note is the contribution of soil fumigation, particularly using the chemical methyl bromide, to agricultural and ecological research. Through comparison of yields to fumigated soil, it has been used to access and optimize the performance of more sustainable crop management practices (Cook et al., 2002). It has also been used in ecological field research to experimentally test ecological theories such as island biogeography (Simberloff and Wilson, 1969).
History The first documented example of soil fumigation using volatile organic compounds dates from 1869, where Baron Paul Thenard applied carbon disulfide (CS2) to French vineyards infested with the root parasite Phylloxera vastatrix (Rohart, 1876; Rommier, 1876). Eventually, over 50 000 ha of European vineyards where fumigated by the end of the nineteenth century. The origin of many chemical fumigants began as waste byproducts of industrial reactions. A mixture of dichloropropene and dichloropropane (DD) was produced as a byproduct of the production of allyl chloride by Shell Company and presented a disposal problem until its nematicial activity was discovered by Walter Carter in Hawaii (Carter, 1943; Caswell and Apt, 1989). The fumigant methyl bromide, initially manufactured as a flame retardant in the 1920s, was produced as a precursor to safer fire retardants such as BCF and Halon 1211. The popular soil fumigant chloropicrin (trichloronitromethane, CCl3NO2) had a more
Figure 1 Traditional shank application of soil fumigants practiced in the southeastern US.
Encyclopedia of Agriculture and Food Systems, Volume 4
doi:10.1016/B978-0-444-52512-3.00250-3
Plant Health Management: Soil Fumigation
insidious origin before its introduction to agriculture. Chloropicrin, produced by the reaction of picric acid and bleach, remained largely a laboratory curiosity until World War I when it gained widespread use in chemical warfare, both a lachrymator and a lethal gas. At the closure of World War I, the US had stockpiled an estimated 36 million pounds of chloropicrin (Roark, 1934) and agricultural scientists begin to assess the surplus chloropicrin for insecticidal properties (Cotton and Young, 1929; Moore, 1918). By the late 1950s, soil fumigation with mixtures of chloropicrin and methyl bromide had become the standard soil disinfestation practice for high-value crops such as tomato and strawberry (Geraldson et al., 1965; Wilhelm et al., 1961), replacing other less effective soil fumigants. For 50 years, methyl bromide was widely used as a soil fumigant due to its low cost, ease of handling, performance under a wide range of soil and environmental conditions, low phytotoxicity, and broad range of activity. Globally, it was viewed as indispensable wherever intensive agriculture was practiced (Vanachter, 1975), and in the US a 1990s economic study estimated that a ban on the use of methyl bromide would cost producers and consumers more than US$1 billion annually (Anonymous, 1993).
Biological, Environmental, and Human Impacts The same combination of acute toxicity and high volatility that make fumigants an effective soil pesticide can also increase the risk of field worker and bystander exposure, contamination of ground and surface water, and environmentally damaging atmospheric emissions, creating a contentious source of conflict between the agricultural industry and consumer and environmental advocates. The registration of ethylene dibromide, at the time one of the most abundantly used nematicides in the world, was suspended by the US Environmental Protection Agency (USEPA) after it was identified in ground waters of Arizona, California, Connecticut, Florida, Georgia, Massachusetts, and Washington (Federal Resister, 1983). A contaminant of DD was detected in municipal drinking wells located near former pineapple production fields on the Hawaiian islands of Kauai, Oahu, and Maui, prompting a voluntary withdrawal by the manufacturer (Oki and Giambelluca, 1987). Use of the popular soil fumigant 1,2-dibromo-3chloroprane (DBCP) was suspended by the USEPA after it was discovered that one-third of male workers at a DBCP manufacturing plant in California were sterile (Thrupp, 1991). Later, a massive sterilization of approximately 1500 banana plantation workers in Costa Rica from DBCP was also documented (Thrupp, 1991). DBCP was also detected in the ground waters of Arizona, California, Hawaii, Maryland, and South Carolina. In 1991, metam sodium gained national attention when a train derailed spilling over 43 000 l into the Sacramento River, causing catastrophic damage to wildlife and the river ecosystem (Cantara Trustee Council, 2007; Brett et al., 1995). Methyl bromide was identified as a significant ozone-depleting compound, and agricultural soil fumigation was implicated as a major anthropogenic source (Yagi et al., 1993; WMO, 1994). Under obligations of the US Clean Air Act and the Montreal Protocol on Substances that Deplete the Ozone Layer (an international stratospheric ozone protection treaty) a
457
scheduled phase-out of production and sale was initiated in developed countries (USEPA, 1993; UNEP, 1995). Since 1999, production and importation of methyl bromide in the US has declined from 25 500 to 434 metric tons. In Europe, registrations for 1,3-dichloropropene (1,3-D) and chloropicrin were canceled and production and sale of both fumigants were phased out between 2008 and 2012.
Current and Future Status In the US, the number of chemical compounds registered for use in soil fumigation has diminished to chloropicrin, dimethyhl disulfide (DMDS), methyl isothiocyanate (MITC) generators (dazomet, metam potassium, and metam sodium), and 1,3-D. Their chemical properties have been extensively reviewed by Ajwa et al. (2003) and Ruzo (2006), and a generalized summary of their maximum-use rates and relative effectiveness to methyl bromide is available (Noling et al., 2010). A comprehensive reevaluation of chloropicrin, DMDS, and the MITC generating compounds by the USEPA in 2010 led to additional regulations place on their use to increase protection of agricultural workers and bystanders. Some of the new regulations include increased buffer zones of untreated soil between the application sites and neighboring residences, detailed site-specific fumigant management plans, emergency preparedness and response notification, and formal applicator training programs administered by the fumigant registrants (USEPA, 2013). The additional compliance assistance and assurance measures will dramatically restrict the ability to apply soil fumigants, particularly in urbanized areas. Recent trends in soil fumigation have focused on developing a set of good agricultural practices (GAPs) for fumigant applicators that, when adhered to, enhance the pest control efficacy of alternative chemical fumigants and minimize their environmental and human health impacts. Key features of GAPs include the use of improved application methods and technology to reduce fumigant application rates, improve the precision of fumigant placement within the soil profile, and restrict their atmospheric emissions. For example, low disturbance fumigant application equipment was developed using small vertical colters mounted forward of a soil bed press to improve the precision of fumigant delivery without disturbing the soil profile (Figure 2). The combined use of low-disturbance application equipment and a soil surface seal reduced cumulative atmospheric emission of 1,3-D by 21% and chloropicrin by 18% when compared to traditional shank applications (Chellemi et al., 2012). An under-bed fumigator using winged steel shanks mounted behind large vertical colters was developed to deliver soil fumigants underneath existing plastic mulched beds without disturbing the integrity of the beds (Figure 3). Significant control of plant parasitic nematodes and weeds with reduced fumigant application rates was achieved when the under-bed applications were made 7 days after planting beds were covered with plastic film (Chellemi and Mirusso, 2006). New formulations of plastic film have been developed and used as tarps to retain fumigants in the soil following application by using layers of ethylene vinyl alcohol resins, nylon, or other polyaminides. A standardized approach for estimating their permeability to soil fumigants was developed and used to assess
458
Plant Health Management: Soil Fumigation
their performance under a range of field and application conditions (Papiernik et al., 2011). Total cumulative atmospheric emissions of chloropicrin and MITC were reduced to less than 5% of the material applied when low-disturbance application technology was combined with improved plastic films (Chellemi et al., 2010). Also included in GAPs are the selective inclusion of specific soil and environmental conditions that mitigate the impact of fumigant emissions. They include soil moisture above field capacity and good soil tilth. Soil tilth is
Figure 2 Use of vertical colters to improve the precision of fumigant placement within the planting bed and to minimize disturbance of the soil profile.
defined as the physical condition of soil as related to its ease of tillage, fitness as a seedbed, and its promotion of seedling emergence and penetration (Karlen, 2005). An illustrated comparison of two fumigated planting beds with contrasting levels of soil tilth is presented in Figure 3. Ranked in order of impact on the retention of fumigants in the soil are soil water content, soil tilth, the type of plastic film, and soil texture (Chellemi et al., 2011). When GAPs are following during the fumigant application process, many soil fumigants can achieve a spectrum of pest control similar to methyl bromide while maintaining a high level of marketable yields (Ajwa et al., 2002; Chellemi and Mirusso, 2006; Gilreath et al., 1999; Locascio et al., 1997; MacRae et al., 2010).
Figure 3 Precision placement of fumigants under plastic mulched beds using vertical colters and winged steel shanks.
Figure 4 A comparison of raised plastic mulched beds prepared in soils with good tilth (left) and poor soil tilth (right).
Plant Health Management: Soil Fumigation
By definition, soil disinfestation procedures are not sustainable because reinfestation of treated soil occurs during crop production, necessitating the reapplication of soil disinfestation procedures for future crops. Future trends in soil fumigation research will include ways to mitigate the impact of chemical fumigants on nontarget beneficial soil microbial communities as a means of extending the time intervals between fumigant applications (Figure 4).
See also: Advances in Pesticide Risk Reduction. Plant Health Management: Crop Protection with Nematicides
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Relevant Websites http://www.mbao.org/ Methyl Bromide Alternatives Outreach. http://ozone.unep.org/new_site/en/montreal_protocol.php United Nations Environment Programme. http://www.epa.gov/opp00001/reregistration/soil_fumigants/index.htm United States Environmental Protection Agency. http://www.epa.gov/ozone/mbr/ United States Environmental Protection Agency.